Gimbal-mounted micromechanical rotation speed sensors possess, for example, two frames with an ability to vibrate with a central inertial mass. Through an electrostatic drive the mass is brought into resonant vibrations about an excitation axis. During a rotation of the sensor about an axis of rotation which is oriented vertically in relation to the excitation axis, the Coriolis force acts upon the oscillating inertial mass. In this way, a vibration is periodically excited about a read out axis which is oriented vertically in relation to the excitation axis and toward the axis of rotation. The amplitude of the oscillation so generated is a direct measure for the rates of rotation to be measured. Reading out the amplitude can take place electrostatically, for example.
Micromechanical rotational speed sensors can, for example, be used in automotive engineering, in aerospace engineering as well as in connection with exploration and production methods. For example, the rotation speed sensors can be used for vehicular stabilization, regulating driving dynamics and for navigation systems or even within systems for autonomous driving. Further possible uses include aircraft navigation and stabilization. In the area of space travel, platforms can be stabilized with such sensors and be controlled in their position. Navigation systems, such as GPS/INS, for example, are supplemented by rotation speed sensors of this type, especially in the area of avionics. In the exploration for raw materials, drill heads, for example, can be controlled with rotation speed sensors. Rotation speed sensors are used for controlling robots in modern production technologies.
In U.S. Pat. No. 4,598,585, a rotation speed sensor with a gimbals-mounted structure is described in which a framework with a vibration capacity is mounted about a y axis. An element fastened on the frame capable of vibration about an x axis is situated inside the frame. An inertial mass is arranged on the internal element. Drive elements serve to set the frame into vibration about the y axis. The displacement of the internal element due to the Coriolis force is measured capacitively.
The known micromechanical rotational speed sensors nonetheless have the disadvantage that the measuring accuracy is often insufficient. Also very large cross sensitivities occur as a rule. Above and beyond this, often a great sensitivity toward vibrations can be established. Furthermore, the known rotation speed sensors are usually associated with high manufacturing costs.
The object of the invention is therefore to create a micromechanical rotation sensor which has a greater sensor sensitivity and a low cross sensitivity and to disclose a method for its' manufacture. Above and beyond this, the rotation speed sensor should be robust, have low sensitivity towards mechanical vibrations, and be economically manufacturability.
This object is accomplished through the micromechanical rotation sensor in and the method for manufacturing a micromechanical rotation speed sensor of the present invention.
The micromechanical rotation speed sensor of the invention includes a first vibration element which is pivoted about a first axis, a second vibration element which is pivoted about a second axis which is oriented vertically in relation to the second axis, an excitation unit in order to set the vibration element into vibrations about the first axis and a read out unit to record vibrations of the second vibration element about the second axis, whereby at least two additional mass elements are fastened on the first vibration element which are aligned symmetrically to a plane which is defined by the first and second axis.
A significantly higher sensor resolution and sensitivity results from the symmetrically aligned additional mass element. Moreover, the added masses or additional mass elements can be extremely large. In this way, there arises a broad transfer of mass centers symmetrically to the axis of rotation, which brings about an extreme heightening of sensor sensitivity. The symmetrical construction reduces cross sensitivity toward rotation speeds outside the axis of sensitivity of the sensor and reduces the sensitivity towards an acceleration acting upon the sensor. The sensor can be manufactured economically and can be designed extremely robustly.
Advantageously, the common center of gravity of the two mass elements lies at the point of intersection of the first and second axis. In this way, a maximal symmetry results.
Preferably, the additional mass elements are manufactured separately from the first and/or second vibration elements, whereby in particular the shape, size or even material of the mass elements are selectively chosen in order to establish the parameters of the sensor. Through the free choice of shape, size and material of the additional masses, the mass distribution, overall mass and the distribution of factors of inertia of the sensor can be selectively chosen. In this way, additional configuration possibilities arise for optimizing the sensor with respect to resolution, cross sensitivity, sensitivity to shock, and reduced influence of manufacturing tolerances or also reduced sensitivity toward vibrations.
In particular, added masses with special physical properties can be formed by freely selecting materials for the additional mass elements which are especially suitable according to the standards of the rotation speed sensor. Furthermore, the sensor element can be trimmed by the special selection of added masses without changes or interferences having to be undertaken on the remaining structures or on the etched out gimbals-mounted structure.
The added masses can be manufactured economically with high precision. Spheres are especially preferably used as mass elements, which can be economically manufactured with a very low geometric tolerance of, for example, 0.1%. In this way, a very high reproducibility of the mass distribution of the rotation speed sensor results through the use of spheres. But rectangular prisms, cones, pyramids or truncated pyramids and cylinders can be used as mass elements which can likewise be manufactured very economically and with low geometrical tolerance. It is especially beneficial to arrange the cones or pyramids with their tips oriented toward one another. In this way, the centers of gravity of the individual mass elements are transferred as far as possible or removed as far as possible from one another.
The additional mass elements in particular have, for example, magnetic properties. This brings about a mutual attraction of the additional mass elements so that they orient themselves completely symmetrically. Further advantages are the adhesion to the substrate resulting from this, the possibility of self-calibration, as well as the possibility of a magnetic or electromagnetic excitation to vibrations.
Preferably the additional mass elements are made of a material which has a higher density than the material of the first and/or the second vibration element. This leads to a beneficial distribution of mass inertia factors. Moreover, metals, for example, especially steel, can be used as material for the additional mass elements, whereas, in contrast silicon, for example, is used for the remaining sensor structure or for the first and second vibration element. There is thus a free choice of material for the added masses, since the material of the added masses need not be compatible with the processing steps, for example, for a silicon wafer, from which the vibration elements, the structure capable of vibration, are advantageously manufactured. In this way, an extreme increase in sensor sensitivity can take place economically.
For example, the first vibration element is a rocker and the second vibration element is a frame, whereby the rocker and the frame form a gimbals-mounted structure capable of vibration which is fastened into a retaining structure.
Advantageously, the rotation speed sensor is manufactured from at least three wafers joined together which are preferably individually processed. Moreover the rotation speed sensor has a bottom wafer, a midsection wafer and a top wafer. In this way, there results a reduced complexity during the manufacturing process as well as the possibility of testing the individual components. Furthermore, the yield is increased owing to which reduced costs arise for the sensors. Moreover, cavity and electrode structures which are situated in the interior of the sensor after assembly can be freely configured. In addition, a greater possibility for selecting electrode material and the material for the added masses exists since the material must be compatible with fewer manufacturing steps. The use of top and bottom wafers of identical construction is possible. This makes possible a completely symmetrical construction in relation to the central plane.
Preferably the first and second vibration element is constructed in the midsection wafer. The midsection wafer can in particular be processed on the upper and underside. In this way, the symmetry of the central plane is guaranteed since the masses or additional mass elements can be applied symmetrically. The temperature drift of the sensor properties can be reduced by the symmetry.
Advantageously, the bottom and/or top wafer are made of alkali-containing glass wafers, such as, for example, Borofloat or Pyrex glass, of which, for example, at least one wafer is provided with an electrode structure. In this way, scattering and cross talk properties are reduced since the electrode structure is situated on insulated material. In particular, for example, the thermal coefficient of expansion is adapted to the silicon of the midsection wafer owing to which thermal distortions can be kept low during manufacture, and from which a reduced temperature sensitivity of the sensor arises during operation. The use of alkali-containing glass wafers in addition makes possible a reliable joining with the midsection wafer using an anodic bonding method.
Owing to the fact that the wafers or the midsection wafer are joined with the bottom and the top wafer, for example, by anodic bonding, there results a reliable connection which requires a temperature of at most 450E C for manufacture. The maximum temperature is low enough so that suitably selected metallic coatings are not altered. That is, no oxidation occurs and no formation of alloys either. The anodic bonding allows a good adjustment of the wafers in relation to one another since no liquid phase arises during the bonding process. The adjustment tolerance of the rotation speed sensor is for this reason mostly less than a few μm.
Advantageously a gap separation is located between the midsection wafer and the bottom wafer or between the midsection wafer and the top wafer which is small in relation to the lateral electrode extension which serves for electrostatic excitation and/or capacitive reading out of the actuator unit and/or sensor unit vibration of the vibration element. The ratio between the gap separation and the lateral electrode extension is, for example, smaller than 1:20, preferably smaller than 1:50, and especially smaller than 1:100 or even 1:1000. In this way, there result very large capacity values which once again make possible high electric signals for the sensor unit or large electrostatic forces for the actuator unit.
Preferably the gap separation for the actuator unit structure, which makes possible the excitation vibration of the first vibration element, is greater than the gap separation for the sensor unit structure which makes possible the read out vibration of the second vibration element. In this way, the actuator unit vibration can take place with a very high mechanical amplitude. In addition, damping the vibration of the vibration with a larger gap separation is less (squeezed film damping), which leads to a higher mechanical amplitude with resonant excitation. On the other hand, a large capacity and therewith a high electrical output signal results through the small slot distance with the sensor element structure.
Advantageously, the wafer on the basis of which the mechanical structure or the first and second vibration element is etched is made of single crystal silicon. Moreover, the structure or gimbal structure of the sensor capable of vibration is etched from a full wafer, that is, manufactured in bulk technology. The structure capable of vibration includes, for example, the first and second vibration element and is preferably constructed on the basis of the midsection wafer. Through the use of single crystal silicon, very slight material damping and furthermore, negligible, slight fatigue and aging phenomena are obtained. Manufacture in silicon technology leads to low manufacturing tolerances with simultaneous low costs. In addition, silicon possesses a high mechanical load-bearing capacity with low density at the same time, from which a robust structure able to bear loads results.
Advantageously, the first and/or second vibration angle is constructed non-rectangularly. That is, the structure capable of vibration has a non-rectangular shape or a symmetrical convex free form. The vibration elements can, for example, be configured round or even have edges, which border on each other at an angle of more than 90E. For example, the vibration elements can be octangular.
In particular, taking into consideration large additional inertial masses, which cause the increase in sensitivity, an enlargement of the capacity surfaces results with higher bending strengths and therewith higher inherent frequencies of the frame or outer structure capable of vibration.
In this way, a high rigidity for setting the rotary band of the frame or the torsion suspension is attained which cannot be attained with a rectangular structure. The torsion frequency is basically determined by the torsion or rotary band as suspension itself. One can consequently greatly shorten the rotary band and attain a Z mode of the sensor adjustable almost independently of the torsion frequency, the sensor being oriented perpendicular to the wafer plane.
Through the shape described above, and through the particular arrangement of the added masses, an especially high sensitivity can be attained with a small construction in connection with a specified surface of the structure capable of vibration.
Furthermore, the inherent frequency spectrum of the mechanical structure is more favorably configured through the non-rectangular shape. Nonrectangular shapes can be found in which the inherent torsion frequencies of the rocker or the structure capable of vibration are the lowest eigenmodes of the structures and all other modes come to lie at significantly higher frequencies. In this way, one can guarantee the required frequency distance between the mechanical noise spectrum, for example in a rough environment, and the operating modes and eigenmodes of the sensor.
Advantageously, the frequency of the actuating unit vibration which is caused by the excitation unit, and/or the frequency of the sensor unit vibration which is generated by the Coriolis force, is the lowest eigenmodes of the structure capable of vibration which is formed by the first and second vibration element. In this way, there especially results a high robustness of the mechanical structure toward shock stresses and mechanical vibrations.
By the use of mechanical damping elements, for example mechanical low pass filters, in constructing the sensor element, it is possible to separate the rotation speed signal from higher frequency noise signals. Moreover the rotation speed signal has a band width from 0 to 100 Hz, for example. Low frequency noise signals, the band width of which is comparable with the band with of the rotation speed, cannot influence sensor behavior, or can only do so with strong suppression, on account of the position of the inherent frequencies of the sensor structure. Moreover, the inherent frequencies of the sensor structure, that is, the actuator unit and the sensor unit vibration, lie at ca. 10 kHz, while all other eigenmodes lie above it.
In particular the surface ratio between the second vibration element and the first vibration element is greater than 5:1, preferably greater than 10:1. Due to this surface ratio between the frame which forms the outer sensor unit structure or the second vibration element, and the rocker which forms the internal actuating unit structure or the first vibration element, there results a further enlargement of the electrical sensor signal with simultaneous optimal mechanical design of the sensor structure or the position of the eigenmodes. Furthermore, a far-reaching decoupling of the inherent frequencies of rocker and frame takes place. Owing to this surface ratio and the ratio of the mass inertia factors going along with it, it is possible to determine the inherent frequency of the rocker basically as a function of the inertial mass or of the mass elements, and the inherent frequencies of the frame basically as a function of the frame geometry. In this way, an almost independent frequency compensation can be attained. That is, the sensor can be determined in a simple manner and with great exactitude.
Preferably the micromechanical rotation speed sensor has a metallic coating for formation of an electrode or electrode structure which is covered with a dielectric layer. In this way, a passivation takes place so that the metallic coating is protected from corrosion. Leakage currents between the insulated electrodes are considerably reduced. Since the metallic coating in particular sits on fixed, unmoved parts of the sensor, there are hardly any restrictions with regard to the type and method of passivation.
Advantageously, the micromechanical rotation speed sensor includes one or more electrodes which are enclosed by a closed printed conductor. Moreover, the printed conductor can, for example, be inherently contacted. Even the corresponding leads can be surrounded by the closed printed conductor. Through this measure, the electric cross talk between the electrodes for the sensor unit and/or for the actuating unit are reduced. Since the metal-coated electrodes sit, for example, only in the top and/or bottom wafer, and in this case not in the structured midsection, the guard electrodes are easy to contact and are subject to fewer frame conditions in their shape and condition than if they had to be installed in the midsection.
Preferably the micromechanical rotation speed sensor has an ohmic pressure contact for connection of the midsection wafer to the bottom wafer or to a bonding pad of the bottom wafer. Even the top wafer can be contacted in this manner. Preferably there is no metallic coating on the midsection wafer. In particular, the entire structure of the midsection wafer possesses an electric potential.
In this way, it is possible for the electrical connection of the midsection wafer to take place through standardized wire bonding pads which have, for example, a size of 100 μm×100 μm. In this way, the entire connection pads can be located on one plane and be arranged lying beside one another. This considerably reduces the expense in electrical contacting of the sensor element with the associated electronics for actuating unit and sensor unit.
Due to the lack of a metallic coating on the midsection wafer, the manufacturing cost of the midsection wafer is considerably reduced. Furthermore, the structure capable of vibration manifests but a very small material damping and has no mechanical distortions. This additionally contributes to a reduced temperature dependency of sensor properties.
The sensor interior can be hermetically sealed, whereby, for example, buried printed conductors serve to contact the electrodes in the sensor interior. Due to the connection between the bonding pads and the electrode surfaces by means of buried printed conductors, the sensor interior can be hermetically sealed and consequently can neither become dirty, nor corrode, nor be changed by moisture or other environmental influences.
The method of the invention for manufacturing a micromechanical rotation speed sensor includes the following steps: Readying at least three wafers; structuring the individual wafers, wherein in one of the wafers a gimbals-mounted structure capable of vibration is mounted; constructing an excitation unit to excite a first vibration of the structure; constructing a read out unit to record a second vibration of the structure, whereby the wafer is connected with the vibration-enabled structure on both sides with a further wafer. Through this method, it is possible to arrange extremely large added masses on the midsection capable of vibration, and in this way to attain a significantly higher sensor resolution.
The further advantages which arise form the manufacture of the rotation speed sensor from at least three individually processed wafers which are subsequently combined were already described in connection with the rotation speed sensor of the invention.
In particular, additional mass elements can be fastened to the axis of the first and/or the second vibration on the structure capable of vibration. Furthermore, the wafer with the structure capable of vibration on its upper and underside can be processed.
Advantageously, the structure capable of vibration or gimbal structure of the sensor can be etched from a single, complete wafer and the suspension of the mechanical or vibration-enabled structure which forms the midsection of the sensor is manufactured in a single etching step. In this way, a high manufacturing exactitude of the geometrical structure or the vibration-enabled structure is obtained, since, for example, several masks need not be adjusted in relation to one another. It is also possible to conduct the structuring of the silicon with flanks perpendicular to the wafer plane. The midsection itself is therefore up-down symmetrical with high exactitude. In this way, a basic source for the “quadrature error” is ruled out. There furthermore results a free ability to configure the lateral shape, especially when using the anisotropic plasma etching technique.
Advantageously a metallic coating structure is applied to the bottom wafer of the three wafers using thin layer technology which, for example, forms condenser surfaces, leads and connection pads. In this way, manufacturing costs are reduced as the complete metallic coating is located on only one wafer. The thin layer technology makes manufacture of small structures with reproducible thicknesses possible which are necessary for a reproducible gap separation. For example, the printed conductor path comes to 10 μm, the conductor path and electrode thickness to 140 nm and the gap separation, for example, to 1.5 μm. The electrical connection of the sensor element to the actuating unit and sensor unit electronics takes place, for example, over standardized wire bonding pads. Their size is, for example, 100 μm×100 μm.
The excitation of the rotation speed sensor or the first vibration element can take place in a great many ways, for example, electrostatically, piezoelectrically, magnetostrictively or even magnetically or using magnetic added masses. In this case, the rotation speed sensor is provided with electrostatic, piezoelectric, magnetostrictive or even magnetic elements or magnetic added masses.
In addition, a control apparatus can be provided which has an electronic unit for regulating and/or forcing the excitation vibration. The electronic unit can be designed such that the first vibration element vibrates in its inherent frequency. It can also be designed such that the vibration of the first vibration element is forced with a certain frequency which need not be the inherent frequency. This has the particular advantage that the read out electrodes for measuring the vibration of the second vibration element do not impair the function of the sensor or falsify the measuring results, even when they are arranged very near to the electrodes for excitation of the first vibration element and/or would influence a free vibration of the first vibration element.
Through the choice of a suitable electrode shape of the excitation electrodes, the influence of the read out electrodes for measuring the vibration of the second vibration element is further minimized. For example, the individual electrodes of a pair of excitation electrodes can be divided and controlled separately by the electronics unit in order to shut off the aforementioned influence or compensate for it.
The reading out process can likewise take place in several known manners and in particular, for example, takes place capacitively or optically. In this case, the rotation speed sensor is provided with capacitive or optical elements for reading out the vibration of the second vibration element generated by the Coriolis force. Continuous automatic testing and automatic calibration functions in operation are possible.